The invention relates to a method for determining an operating point of a work machine and/or of an asynchronous motor driving the latter, a power input of the work machine and/or its delivery rate characterizing an operating point, one or more operating point-dependent measurement variables of the work machine being detected by one or more sensors, and the measurement values being evaluated and/or stored while the work machine is in operation. The invention relates, further, to a method for monitoring an operating point. The invention relates, furthermore, to an apparatus for carrying out the method.
In order to ensure that a work machine operates reliably and efficiently, its operating point must be known.
When a pump arrangement, in particular a centrifugal pump arrangement, composed of a pump and of an asynchronous machine driving the latter, is in operation, evidence of its operating point is often required. The operating point of a working turbomachine, in particular a centrifugal pump, on its delivery flow/delivery head characteristic curve or Q-H characteristic curve, is characterized in particular by its delivery flow, also hereafter called the delivery rate. There are various possibilities for determining this. It can be determined by measuring the delivery flow or by pressure measurement. In the latter case, the difference in pressure between the delivery side and suction side of the pump is usually measured. The delivery head is estimated as the quotient of the pressure difference, density and gravitational acceleration. In the case of water as a delivery fluid, a pressure difference of 1 bar corresponds to a delivery head of approximately 10 meters. Furthermore, an operating point of a centrifugal pump is determined by electrical measurement, the motor power output being calculated from current and voltage measurements, taking into account the efficiency of the motor.
Direct measurement of the delivery rate usually requires magnetoinductive flowmeters. Indirect determination of the delivery rate arithmetically presents additional difficulties. If, for example, a delivery rate is derived from the values of a delivery flow/delivery head characteristic curve, a Q-H characteristic curve, in which the delivery head H is plotted against the delivery flow, or of a delivery flow/power characteristic curve, a Q-P characteristic curve, in which the power P is plotted against the delivery flow Q, this is difficult or even impossible in those situations where there is a flat or a discontinuously rising Q-H characteristic curve or Q-P characteristic curve. If the delivery rate is to be determined by means of measured pressures from the Q-H characteristic curve of a centrifugal pump, the Q-H characteristic curve must be unequivocal, that is to say a Q value must be assignable exactly to each H value. This condition is often not fulfilled in practice. Q-H characteristic curves are either too flat or even ambiguous. The same problem also arises when the delivery flow Q is to be determined by means of a measured power input from the delivery flow/power characteristic curve, the Q-P characteristic curve. The profile of the Q-P characteristic curve is also often flat or even ambiguous.
A combination of the above methods is known from WO 2005/064167 A1. This entails a considerable outlay in measurement terms, since both the differential pressure of the pump and electrical power have to be measured.
Measuring the electrical power input of a motor/pump assembly entails a certain amount of outlay in practice. Active power measurement takes place in a switch cabinet, takes up space there, particularly for measuring the motor current by means of current transformers, and necessitates an outlay in assembly terms which has to be performed by specialized electricians.
An arrangement and a method for determining the power and/or torque of induction motors are described in DD 258 467 A1. A proximity switch is arranged on the rotor of an induction motor for the purpose of detecting one or more pulses per revolution of the motor shaft, and a pulse shaper stage for detecting the synchronous rotational speed from the line frequency is connected between the network and a microcomputer. In addition, the arrangement has a device for detecting the temperature of the motor and a microcomputer in which all the measurement data are acquired and evaluated for the purpose of regulating the further process sequence. The power and/or torque of the induction motor are/is determined from the time of one or more periods of the motor rotational speed and one or more periods of the synchronous rotational speed. The power and/or torque of the induction motor are/is determined by counting the pulses of the motor shaft within what is known as a gate time which is fixed by one or more periods of the synchronous rotational speed. The “Kloss equation” is used for determining the power and/or torque. The method requires a plurality of input variables, one of which is also the synchronous rotational speed which is determined from electrical measurement variables. In addition, the results have to be corrected as a function of the operating temperature of the motor, thus making it necessary to determine and store required correction factors per motor type by measurement beforehand. This arrangement has a complicated configuration. This method has proved to be unsuitable in industrial practice. It is a particular disadvantage, even when the active power input of an asynchronous motor is measured conventionally by active power meters and current transformers, that it is absolutely necessary that such an arrangement is installed by specialized electricians.
US 2007/239371 (=DE 10 2006 049 440) discloses a method for detecting an operating state of a pump, in particular of a centrifugal or positive displacement pump, in a pump plant. The method and its device serve for detecting a faulty operating state of a pump, pump plant and hydraulic plant, as compared with a stored normal state. A pressure sensor detects the pressure time profile in the delivery medium. A calculated characteristic value characterizes the pulsation of the pressure and/or flow profile in a calculation time interval. By the calculated characteristic value being compared with at least one stipulated characteristic value or with a characteristic value range delimited by this, the stipulated characteristic value or the characteristic value range delimited by this corresponding to a relevant operating state of the pump, the operating state is determined and output. In the case of a diagnostic appliance with a connected pressure sensor and with an additional oscillation sensor, the rotational speed of the pump is determined from the pressure sensor signal and is supplied to the oscillation sensor. The reasons for this are not disclosed. Neither the rotational speed information nor any other variables give evidence of the operating point on a Q-H or Q-P characteristic curve and/or the power input at which the pump is operated. Only deviations from predetermined and stored reference values are indicated by this method.
DE 196 18 462 A1 discloses a further method and a device for determining an extrinsic power parameter of an energy-converting device, such as the volume or mass throughflow through a motor-driven centrifugal pump, in which an operating state-dependent intrinsic variable is continuously determined.